The present invention relates to a WC-based composite ceramics sintered body used in sliding wear parts of a machine, edged tools for cutting polymer materials such as polyethylene or metals such as aluminum alloys, press dies for bending and cutting metals or polymer materials, or the like which are required to have a high wear resistance.
Heretofore, most wear-resistant members, such as sliding wear parts of a machine, cutting blades or dies, have been formed by using ferrous metals represented by die steel. However, ZrO2-based or Si3N4-based ceramics has been recently used as a substitute for such ferrous metals having an insufficient wear resistance.
Particularly in press working required to provide an accurate size of a finished product, since a press die tends to incur seizing with a workpiece and earlier wear-out, it is necessary to form the press die with a material having a long life and excellent wear resistance.
As a ceramics die material having a higher hardness and superior seizing resistance in comparison with metals, Japanese Patent Publication No. Hei 01-024747 (Japanese Patent Laid-Open Publication No. Sho 58-120571), Japanese Patent Laid-Open Publication No. Hei. 03-051667 and Japanese Patent Laid-Open Publication No. Hei 09-221352 disclose die materials in which ZrO2 ceramics is strengthened by dispersing carbide or nitride grains of transition metals therein.
Required properties of the die material in case of applying ceramics to sliding wear parts, cutting blades or dies include four parameters of Young""s modulus, hardness, heat conductivity and electrical workability.
As to Young""s modulus, conventional ferrous metals represented by die steel or conventional ceramics-based materials including zirconia as a primary component for providing high strength and toughness have a low Young""s modulus of 2.5 GPa or less. Thus, each conventional material is insufficient in stiffness during its high-speed sliding motion with respect to an opposite member. This causes an elastic deformation of the material and excessive friction between the material and the opposite member, resulting in an abnormal wear of the material. Specifically, the ferrous materials cause an abnormal wear, while in ZrO2-based materials, the resulting frictional heat promotes crystal transformation and thereby an undesirable wear arises from fall-off of grains. This wear has an influence on usable period or life of parts. In particular, such wear would be a fatal factor in a field having a requirement of accurate motion.
As to hardness, the advantage of ceramics compared to conventional metal-based materials is to provide an improved wear resistance based on high hardness. However, a ceramics-based die material including zirconia as a primary component is insufficient in hardness and thereby low wear resistance or short life is undesirable exhibited.
WC includes free carbon. Thus, during a sintering process in an inert atmosphere, the free carbon remains in a sintered body, and undesirable fracture can originate in the remaining free carbon, resulting in lowered strength of the sintered body. In a ZrO2-WC composite body, ZrO2 and WC have different thermal expansion coefficients and are oxide and carbide, respectively, which are not solid-soluble each other. Thus, at a stage in which the composite body has been cooled from a high temperature in a sintered state down to a room temperature, cracks can arise from a difference in thermal expansion, and the composite body tends to fracture due to lowered mechanical properties, particularly in bending strength.
As to heat conductivity, when heat conductivity is low, heat generated by friction during high-speed motion can be accumulated in the material, and this accumulated heat causes phase transition in zirconia as the primary component, resulting in fracture due to the transformation.
As to electrical workability, in parallel with advancing electrical machining technologies, such as a wire-cut electric discharge machining, as well as mechanical machining technologies, many parts are recently produced by using such technologies, and it is often the case that an electrically machined surface of such parts is used as it is. However, even in the zirconia-based ceramics material enhancing hardness and conductivity, its electrically machined surface has a degraded surface roughness due to insufficient fineness and uniformity of the internal structure of the die material. As a result, the electrically machined surface could not be used as it is.
It is an object of the present invention to provide a ceramic sintered body capable of sufficiently fulfilling various properties, such as Young""s modulus, hardness, heat conductivity and electrical workability, which are required as materials for sliding parts, edged tools and dies.
The present invention provides a WC-based composite ceramics sintered body consisting of 40 volume % to 90 volume % of WC including solid-solved oxygen represented by a chemical formula of WCxOy (where 0.005 less than y/x+y less than 0.05), with the remainder being partially stabilized ZrO2 and inevitable impurities, wherein the partially stabilized ZrO2 includes one or more of stabilizers selected from the group consisting of Y2O3, CeO2 and MgO.
The WC-based composite ceramics sintered body can be obtained by the following process. A pre-sintered prepared powder of raw material is held in an atmosphere containing oxygen in a temperature range of 200xc2x0 C. to 600xc2x0 C. for 0.1 hour to 3 hours to provide WC grains including solid-solved oxygen represented by a chemical formula of WCxOy (where 0.005 less than y/x+y less than 0.05). As a phase for filling up the grain boundary of the WC grains, partially stabilized ZrO2 is used which includes one or more of stabilizers selected from the group consisting of Y2O3, CeO2 and MgO. Then, the obtained WC is subjected to a normal sintering process or hot press sintering process in an inert gas in a temperature range of 2023 K to 2123 K to obtain the WC-based composite ceramics sintered body. Simultaneously, through the oxidizing heat treatment, free carbon in the WC grains can be removed to provide an improved grain fall-off resistance, and an enhanced hardness, toughness and strength of the composite material. Further, the electrical machining operation can provide an improved electrical workability and heat conductivity with lower electric resistance.